A water-cooled outboard engine is customarily equipped for an overheat preventing device with a temperature sensor for detecting the temperature of the engine so that, when the engine temperature exceeds a predetermined level, the temperature sensor operates a warning device such as a buzzer. However, the overheat preventing device of the prior art has the disadvantage that seizure of the engine has often started when the warning device is operated, (although this depends upon the preset temperature of the temperature sensor), and that the overheating of the engine proceeds in case the warning sound of the buzzer has not been heard or heeded.
Therefore, there has been proposed an improved system, as disclosed in Japanese Patent Application No. 55-49306 and U.S. patent application Ser. No. 249,713, filed Mar. 31, 1981. According to this improved system, based upon the observation that overheating of the engine is caused by an insufficient supply of the cooling water, when the pressure of the cooling water becomes lower than a predetermined level, overheat is detected by a water pressure sensor which actuates a warning device. Also, when the engine temperature becomes higher than a predetermined level, this fact is detected by a temperature sensor which actuates means automatically to stop the engine.
According to the aforementioned system, however, because the water pressure sensor operates the buzzer when the pressure of the cooling water is lower than the predetermined level, the buzzer generates its warning sound even during a low output and slow running operation of the engine such as the idling operation or the trawling navigation, for example, because the water pressure is low as a consequence of the fact that the RPM of the cooling water pump is low. The continuous sound of the buzzer during the trawling navigation is discordant, although the sound during idling operation can be endured. Therefore, the operating pressure of the water pressure sensor has to be set lower than the highest idling pressure, and this is quite low.
In case such low pressure is used as the operating pressure of the water pressure sensor, the engine overheat has usually already proceeded to a considerable extent when the water pressure sensor actuates the alarm when the water pressure is low as the consequence that the inlet or conduit of the cooling water is clogged, or the cooling water pump malfunctions during high speed running operation. As a reuslt, at the time when the water pressure sensor operates, the temperature sensor also operates, thereby affording no time after the buzzer sounds before the engine is stopped. This invites the concern that the engine will be stopped without taking any countermeasure by looking for the cause of the drop in water pressure, and correcting that cause.